The present invention relates to a vacuum interrupter, particularly to a vacuum interrupter of which a vacuum envelope comprises a metallic member and an insulating ceramic member.
As shown in FIG. 1, a vacuum envelope of a conventional vacuum interrupter comprises a circular insulating cylinder 1 as an insulating member, and circular metallic end plates 2 and 3 as metallic members. The insulating cylinder 1 is made of insulating ceramics such as alumina ceramics glazed on an outer surface, or of crystalized glass. The metallic end plates 2 and 3 are made of a Fe-Ni-Co alloy or a Fe-Ni alloy.
An electric stationary lead rod 4 is secured to the metallic end plate 2 coaxially to the insulating cylinder 1 in such a manner that it enters in the insulating cylinder 1 in vaccum-tightness via a circular aperture at a center of the one metallic end plate 2. On an inside end of the stationary lead rod 4, a stationary contact 5 is secured.
An electric movable lead rod 6 is inserted in the insulating cylinder 1 via a circular aperture at a center of the other metallic end plate 3. The movable lead rod 6 is supported in vacuum-tightness by bellows 7, being coaxial to the insulating cylinder 1. On an inside end of the movable lead rod 6, a movable contact 8 is secured.
The stationary and movable contacts 5 and 8 are separable by an axial movement of the movable lead rod 6.
A substantially circular cylindrical arc shield 9 is provided coaxially to the insulating cylinder 1 around the stationary and movable contacts 5 and 8, projecting from the one metallic end plate 2. The arc shield 9, which is made of stainless steel or iron, serves to protect an inner surface of the insulating cylinder 1 from the arcing products generated between the stationary and movable contacts 5 and 8.
In case the insulating cylinder 1 is made of insulating ceramics glazed on the outer surface, it is manufactured through the following steps. The manufacturing steps comprise the step in which a raw materials of glaze is applied by conventional method to an outer periphery of a cylinder made of unglazed insulating ceramics, and the subsequent step in which the cylinder applied with glazing materials is fired to change the applied glazing materials into an impervious glassy film of glaze. This glassy film is capable of protecting the outer periphery of the insulating cylinder 1 from adsorbing moisture and pollutant, thereby preventing an insulating performance of the outer periphery of the insulating cylinder 1 from being reduced.
On the other hand, both of annular end surfaces 1a and 1b remain unglazed even in the course of forming the glassy film of glaze on the outer periphery of the insulating cylinder 1 by firing.
Metallized layers (not shown) are formed on the unglazed annular end surfaces 1a and 1b, respectively. The metallic end plates 2 and 3 are directly brazed by conventional method to the metallized layers, respectively.
There are significant problems described hereinafter in manufacturing the insulating cylinder 1, generally, insulating members for a vacuum envelope of a vacuum interrupter.
Since the above glazing materials include some high vapor pressure components such as anhydrous boric acid B.sub.2 O.sub.3, sodium monoxide Na.sub.2 O and potassium monoxide K.sub.2 O, they are vaporized in a step of vacuum brazing at a temperature between 900.degree. C. and 1050.degree. C. in a vacuum furnace to be deposited on vacuum-room-side surfaces of the insulating members for the vacuum envelope, which reduces an insulating performance of the vacuum-room-side surfaces of the insulating members, and to be deposited on surfaces of interiors of the vacuum furnace, which causes to hinder the continuing operation of the vacuum furnace.
Additionally, when a temporarily assembled vacuum interrupter is set with the glassy film of glaze coated on the outer periphery of the insulating member, in the vacuum furnace, there are inconveniences due to the glassy film of glaze coated thereon, in handling a jig for holding the temporarily assembled vacuum interrupter because the glassy film softens in the brazing process.
Additionally, in the process of making the glazed ceramics through which the glazing materials applied thereto is changed by firing into a glassy film, then, the glazed ceramics coated with glassy film being cooled, there are tendencies to twist the glazed ceramic insulating member for the vacuum envelope and to crack the glassy film, due to the inconsistency of thermal expansion coefficients between the unglazed insulating ceramics and the glassy film of glaze.
In conclusion, many technical difficulties lie in the process that an impervious glassy film which is changed from glazing materials by firing is formed in mass production, high quality and high yield on an atmospheric-side surface of an insulating member for a vacuum interrupter. The technical difficulties and expensiveness of the glaze make the insulating member for the vacuum interrupter considerably expensive.
Even when the desired glassy film is sucessfully formed on an atmospheric-side surface of an insulating member for a vacuum envelope, the following problems are incidental to the full manufactured vacuum interrupter. Metallized layers, as metallized layers (not shown) on the annular end surfaces 1a and 1b, are formable on a surface of an unglazed portion of insulating ceramics of an insulating member for a vacuum interrupter, but not formable on a glassy film of glaze. Therefore, between the metallized layers on the surface of the unglazed portion of insulating ceramics of the insulating member and the glassy film of glaze on the atmospheric-side surface thereof, a gap of unglazed surface is inevitably formed, which causes to expose the surface of the unglazed insulating ceramics to the atmosphere. The surface of the unglazed insulating ceramics adsorbs moisture and pollutant, which causes insulating performance of the insulating member for the vacuum interrupter to lower, and which causes a brazed portion and its vicinity between the insulating member and the metallic member of the vacuum envelope to be corroded to a vacuum leakage of the vacuum envelope, resulting in the lowered reliability of the vacuum interrupter.